The reason I ask is because your gmap3d file is dated 5/22/19. If that's when you installed the maps, all well and good. However, if you have installed 2020.2 that date should be later, I think. In that case, you might have some remnant files from the last time that Garmin Express was able to put the files on your zumo's internal memory.
Getting back to the 660, then, what I would do with the issue is
a) Find a route that makes it lock up when you do a preview.
b) Back up the gmap3D file from 5/22
c) Backup the entire zumo then delete the gmap3D file from the zumo.
d) Pull the SD card
e) Use MapInstall to load only your riding region to the zumo's internal memory, shooting for a mapset of about 175Mb
f) See if the 660 will do a preview without locking. See if it will "Scroll the route to the end of the route and zoom in" without locking or crashing.
One the 665 it would be interesting to see what Express shows when you go to "Tools & Content>Utilities" and scroll down to the usage. It would also be interesting to see if it has a large map file that is in the zumo>Garmin directory that is chewing up memory.
RE: One the 665 it would be interesting to see what Express shows when you go to "Tools & Content>Utilities" and scroll down to the usage. It would also be interesting to see if it has a large map file that is in the zumo>Garmin directory that is chewing up memory.
I have never looked at the "Tool Content - Utilities" (at least that I remember) Will give it a look see and report back.
Update. I did look for the "Tool Content - Utilities" (in Garmin Express) and could not find the reference but I was also updating maps so I may have missed it. But I am using the MAC version.